Method and apparatus for spray coating of articles



July 10, 1956 E. M. RANSBURG METHOD AND APPARATUS FOR SPRAY COATING OF ARTICLES Filed Nov. 30, 1951 YIIIIIIII I I2 Edwin M. Ronsbur INVENTOR.

ATTORNEY United States Patent METHOD AND APPARATUS FGR SPRAY COATING 0F ARTICLES Edwin M. Ransburg, Indianapolis, Ind, assignor to Ransburg Eiectro-Coating Corp., a corporation of Indiana Application November 30, 1951, Serial No. 259,257 11 (Zlaims. (Cl. 117-93) This invention relates to spray coating of articles and more particularly to the electrostatic deposition of liquid coating material on articles.

This invention is particularly adaptable for use in the electrostatic deposition of coating material in atomized form upon articles having an abrupt discontinuity. In the coating of such articles by exposing them to the spray issuing from a spraying device in an electrostatic coating apparatus Where deposition of the coating material particles is primarily the result of electrostatic force and where the particles are given an electric charge at the time they leave the spraying device or shortly thereafter, it is, of course, necessary that sufficient quantities of the coating material be suppied in spray form to coat the article as completely as desired up to the discontinuity It has heretofore been the practice to move the article through the coating zone, which may be defined as the zone defined by the electrostatic field created between the electrode carrying the high potential and the article, at a rate which is substantially constant and to continue the rate of feed of coating material to the atomizing means at a constant rate until the discontinuity has been fully coated. If the arrangement for moving the article through the coating zone is such that a time lapse occurs between the conclusion of the coating of one article and the commencement of the coating of the next, any coating material supplied to the atomizing means during that time period is wasted. If during such period no article is in the coating zone the atomized particles are merely lost. However, if the article which has just been completely coated is still within the coating zone the atomized particles will continue to be deposited thereon resulting in an overcoating of the article which is not only wasteful but also may cause the coating to sag or run.

The foregoing problems have arisen principally because of the difficulties encountered as a result of the normally relatively slow termination of discharge from the atomizing means. This dificulty is particularly pronounced Where the atomizing means is of the electrostatic rotary conical type disclosed and claimed in the copending application of E. M. Ransburg filed in the United States Patent Office on February 13, 1950, as Serial No. 143,994. The electrostatic atomizer disclosed in said copending application, While providing an extremely eflicient electrostatic coating system, presents certain problems when it is desired to achieve an instantaneous shutoff. Thus even if the feed of coating material to such an atomizing head is cut oif abruptly there still exists coating material on the inner surface of the atomizing head which, by rotation of the head, is caused to flow to the atomizing zone of the head (which is in the form .of .an annular discharge edge), and until substantially all of such material has flowed to the edge atomization of coating material continue, even though at a diminishing rate.

An example of the problems encountered, which is described in detail hereafter, is when such atomizing 2,754,227 Patented July 10, 1956 heads are used to coat the internal surfaces of a hollow object such as a cylinder. It has heretofore been the normal practice in coating such surfaces to insert the atomizing head into the cylinder and to operate it as it is passed into, through and out of the opposite end thereof. As the feed of coating material is continuous until the head has passed through and out of the opposite end of the cylinder, the entire interior surface of the object is uniformly coated but the material atomized from the head after it has passed out of the end of the cylinder is wasted.

The present invention provides a method and apparatus for avoiding waste not only in coating objects of the type just described but when coating any object having an abrupt discontinuity, such as the edge of a fiat surface.

According to this invention the speed of relative move ment between the atomizing means and the article being coated is varied and such variation is accompanied by a proportionate variation in the rate of feed of coating material to the atomizing zone. Thus it is possible to shut off the means which supply coating material to the atomizing head at a point in the operation Where coating is still continuing with the result that the flow of coating material to the atomizing zone is progressively decreased. While this Would normally result in an underspray of all areas subsequently entering the coating zone, such underspraying is avoided by correlating the speed of relative movement between the atomizing means and the article and the area still to be coated with the decreasing rate of flow to the atomizing zone. Thus if the area undergoing coating is unchanged, the speed of relative movement is decreased, or if the area remaining to be coated is of decreasing extent (such as is encountered where the article is tapered at one end) the speed of relative movement may be unchanged. Means are provided for initiating the cutoff of feed to the atomizing head at a point where the coating material remaining on the head to be flowed to the atomizing zone with continued rotation of the head is suflicient to provide a coating of the desired thickness on the remainder of the artiole. Thus, at approximately the time the discontinuity of the article is fully coated the supply of coating material available to the atomizing zone is substantially completely depleted so that no further atomization takes place. In this manner waste of coating material is avoided.

The foregoing and other features of this invention will be apparent from the following description and drawing, in which the figure is aside elevation, partly in section and partly broken away for clarity of illustration, of a coating apparatus embodying the invention.

While one form of apparatus is illustrated in the accompanying drawing, and will be described hereafter as providing means for correlating the relative movement between the atomizing head 'andfthe article with the rate of feed of coating material to the head, it will be understood that this is'a representative embodiment only. It will also be understood that forms of spraying devices other than the particular electrostatic atomizing device shown may be utilized'and that other apparatus may be employed for accomplishing the purposes and for practicing the methods of the invention and it is to be understood that other embodiments may be utilized without departing from the contemplated scope of the present invention and that no. limitations are to be implied from the specific description now provided.

Referring now to the drawing, there is shown an apparatus which may be employed for coating the interior of cylindrical objects, such as the water heater jacket 10. The cylinder is mounted upon a support 11 suitably grounded at 12 and positioned adjacent the coating apparatus. The latter includes a supporting frame, indicated generally at 13, supported in spaced relationship above a base14 by columns 15 and carries at its upper end an electric motor 16 for driving the various parts of the apparatus. Rotatably mounted on bearings 17 and 18 provided in the upper and lower parts of the frame is a drive shaft 19 provided with a spiral thread 20. It will be noted that the upper part of the thread 20a is formed with a substantially constant pitch while the lower part 2% of the thread, that is the portion below the point indicated at 21, is provided with a smaller pitch. The pitch in the portion 20b progressively decreases and terminates in a return thread 200 which is of comparatively larger pitch and extends from the lower to the upper portion of the drive shaft.

A bevel gear 22 is freely rotatable on the upper portion of the drive shaft 19 and meshes with a second bevel gear 23 fixed to the shaft of the motor so that the gear 22 idles with operation of the motor. Means in the form of a magnetic clutch 24 is provided for clutching the shaft 19 to the gear 22 to rotate the shaft, the magnetic clutch being actuated through a manually operable switch 25 connected on one side to a suitable source of current and connected by the brushes 26 and 27 to the electrical system of the magnetic clutch. The arrangement is such that manual operation of the switch 25 energizes the clutch to clutch the shaft 19 to the gear 22. Slidably mounted on the shaft 19 is a carriage in the form of a cast housing 28 provided with suitable slide bearings 29 and 3t sliding on the shaft 19 and with a key 31 engaging the threads 20 in the shaft so that rotation of the shaft produces vertical reciprocatory movement of the carriage. The carriage is provided with an integral annular flange 32 in which is fixed a carrying sleeve 33 made of insulating material. Fixed to the outer end of the sleeve is a hollow body member 34 which rotatably supports a hollow tube 35 which carries at its lower end an atomizing means provided with an atomizing zone, that is, a zone or area from which atomized particles of coating material are emitted. In the embodiment chosen for illustration the atomizing means is of the type described in the said copending Ransburg application and includes an atomizing head 36 and an annular discharge edge 37 with the discharge edge being the zone from which atomization takes place. Fixed to the shaft 35 is a bevel gear 38 which meshes with a bevel gear 39 fixed to the outer end of a shaft 40. The shaft 40 is, like the sleeve 33, made of insulating material and is rotatably supported in the sleeve by a pair of antifriction bearings 41. The inner end of the shaft 40 carries a bevel gear 42 meshing with a bevel gear 43 provided on a vertical spline shaft 44 mounted for rotation in the frame 13 on upper and lower bearings 45 and 46. Adjacent the upper end of the spline shaft 44 there is provided a bevel gear 47 which meshes with the gear 23 so that the spline shaft is constantly rotated with operation of the motor 16. The gear 43 is annular and provided with internal teeth meshing with the teeth 48 of the spline shaft in a manner permitting free sliding movement of the gear 43 on the shaft. To this end the housing 28 is provided with a key 49 which is received in an annular groove 50 provided on the gear 43 so as to retain the gear in fixed relationship with the housing while permitting it to rotate with rotation of the spline shaft.

For supplying coating material to the atomizing head 36 there is provided a pump 51 driven by a suitable motor 52 and connected by means of a flexible hose 53 to the end of the hollow shaft 35. Coating material from the pump is fed through the hollow portion 54 of the shaft 35 which opens at the center of the annular head 36. Because of the fact that the sleeve 33 and shaft 40 are of insulating material, it is possible to maintain the entire atomizing head assembly at a high voltage with respect to surrounding objects by connecting it with a suitable high voltage source 55. One end of this high voltage housing reaches its starting source is connected by the lead 56 to the body portion 34 (which being of metal is electrically connected to the head 36) and the other end of the source is grounded at 57.

Provided on the frame 13 at a point approximately opposite the point 21 of the shaft 19 there is provided a switch 53 which is connected by suitable Wiring to the pump 51 and adapted to be operated by the housing 28 as the latter is moved downwardly with rotation of the shaft 19. Adjacent the upper portion of the frame there is provided another switch 59 connected to the magnetic clutch 24 by the brushes 26 and 27 and adapted to be operated by the carriage as it moves upwardly to the position of the switch 59 to deenergize and hence to halt rotation of the shaft.

The apparatus just described is operated in the following manner. Current is supplied to the motor 16 to rotate the spline shaft 44 and hence rotate the atomizing head 36. The pump 51 is started to supply coating material to the head and when the supply has reached the head and has been flowed to the edge by the rotation thereof, the manual switch 25 may be operated to clutch the drive shaft 19 to the motor 16. With rotation of the drive shaft 19 the housing 28, and hence the atomizing head 36, is moved downwardly and during such downward movement coating material is electrostatically atomized in the manner set forth in the above mentioned copending Ransburg application from the atomizing edge 37 and is deposited upon the interior of the cylinder 10. When the carriage reaches the point 21, switch 58 is operated to shut off the pump 51 whereupon feed to the coating head is halted. Coating material previously fed to the head 36, however, is still flowed outwardly toward the edge 37 by the continued rotation of the head although the amount of material which flows to the atomizing edge progressively decreases after the pump has been shut off. The housing 23, however, upon reaching the point 21 and operating the switch 58 is thereupon moved at a progressively slower rate due to the engagement of the key 31 with the spiral thread 20b beneath the point 21 which, as prevously explained, has a progressively smaller pitch. The pitch is chosen so as to correlate the movement of the head 36 with the decreasing supply of coating material to the atomizing edge so that the coating thickness deposited on the interior of the cylinder during the final portion of travel of the head is substantially equal in thickness to the coating material deposited during the initial part of the coating operation. The point at which the switch 58 is located and the point at which the spiral thread on the drive shaft acquires the smaller pitch are so arranged that at the time the entire interior surface of the cylinder has been coated and the head 36 passes out the lower end of the cylinder, flow of the coating material to the atomizing edge has substantially stopped. At this point, where the housing 28 reaches the lowermost portion of the spiral thread, the key 31 is engaged by the return thread 200 rapidly to return the housing towards its starting position at the upper part of the frame. As the position it contacts the switch 59 which, as previously noted, deenergizes the magnetic clutch 24 to halt rotation of the drive shaft 19. When this occurs the operator may remove the cylinder 10 and replace it with another if such be the arrangement or the conveyor upon which the cylinder is mounted may be moved so as to bring another cylinder into position to be coated. 'A subsequent cycle is initiated again by depression of, the switch 25.

While in the foregoing description the switch 58 has been described as connected to the motor 52 for shutting off the pump, it is clear that other arrangements may be used, for example the switch may operate a signal, such as alight, which indicates'to the operator that the flow from the pump should be shut off. This latter step could, of course, be accomplished not only by shutting on the'motor 52 but by valve means which bypass the coating material moved by the pump.

While I have shown and described a particular embodiment of my invention, it is to be understood that it is capable of many modifications. Changes, therefore, in the construction and arrangement may be made without departing from the spirit and scope of the invention as disclosed in the appended claims.

Iclaim:

1. The method of coating an article by electrostatic deposition of coating material from an atomizing means comprising feeding coating material from a remote control point to the atomizing zone of the atomizing means where said material has an exposed surface, creating an electrostatic held between the atomizing zone and the article of sufiicient strength as to be capable of electrostatically atomizing coating material at said zone and electrostatically depositing the atomized coating material on the article, causing relative movement between the article and the atomizing means to coat a portion of the article and then reducing the feed at the control point to progressively reduce the rate of feed to the atomizing zone while progressively reducing the speed of relative movement to coat another portion of the article.

2. The method of coating an article by electrostatic deposition or coating material from an atomizing means comprising feeding coating material at a substantially constant rate to the atomizing zone of the atomizing means where said material has an exposed surface, creating an eiectros field between the atomizing zone and the article of sufficient strength as to be capable of electrostaticall atomizing coating material at said zone and electrostatically depositing the atomized coating material on the article, causing relative movement between the article and the atomizin means to coat a portion of the article and then changing the rate of feed to the atomizing e and substantially simultaneously correlating the speed of relative movement to the changed rate of feed to minimize undesired variations in coating thickness.

3. In a device for electrostatically coating an article, an atomizing means having an atomizing Zone, means for feeding coating material to the atomizing zone, a movable carriage carrying the atomizing means, a rotatable drive shaft having a spiral thread of varying pitch, means on the carriage engaging the thread to move the atomizing means at varying speeds corresponding to the pitch of said thread relative to an article to be coated with rotation of the drive shaft, and means for varying the rate of feed of coating material to the atomizing zone to supply coating material to the atomizing zone in a quantity which varies substantially directly as the speed of relative movement.

4. In a device for electrostatically coating an article, an atomizing means having an atomizing zone, a movable carriage carrying the atomizing means, a rotatable drive shaft having a spiral thread of constant pitch along one portion and a pitch progressively diminishing to zero at another portion, means on the carriage engaging the thread to move the atomizing means at a constant speed during engagement with the first portion of the thread and to move the atomizing means at a progressively slower rate during engagement with the second portion of the thread, means for feeding coating material to the atomizing zone at a constant rate during said engagement with the fhst portion and means for progressively reducing to zero the feed to the atomizing zone during said engagement with the second portion.

5. Apparatus for electrostatically coating an article comprising a frame, a vertical drive shaft rotatably mounted in the frame and provided with a spiral thread having a constant pitch along its upper portion and a progressively smaller pitch along its lower portion, a carriage movably mounted on the frame and provided with means engaging the thread to move the carriage along the drive shaft with rotation of the shaft, a rotatably mounted annular atomizing head carried by the carriage and provided with an outer annular discharge edge and a central feed opening, a pump for supplying coating ma= terial to the feed opening, means for rotating the head to flow coating supplied thereto to the discharge edge, means for rotating the drive shaft to move the carriage at a rate corresponding to the pitch of the thread, and means operated as the engaging means on the carriage engages the thread on the lower portion of the drive shaft to render the pump inoperative to supply coating to the feed opening whereby the flow to the discharge edge is progressively reduced substantially to zero during the decelerated movement of the carriage.

6. Apparatus for electrostatically coating an article comprising a frame, a vertical drive shaft rotatably mounted in the frame and provided with a spiral thread having a constant pitch along its upper portion and a progressively smaller pitch along its lower portion, a carriage movably mounted on the frame and provided with means engaging the thread to move the carriage along the drive shaft with rotation of the shaft, 21 rotatably mounted annular atomizing head carried by the carriage and provided with an outer annular discharge edge and a central feed opening, a pump for supplying coating material to the feed opening, means for rotating the head to flow coating supplied thereto to the discharge edge, means connected to said head for creating an electrostatic field between said head and an article of suflicient strength to be capable of electrostatically atomizing coating material at said edge and electrostatically depositing the atomized particles on the article, means for rotating the drive shaft to move the carriage at a rate corresponding to the pitch of the thread, and means for creating a signal as the engaging means on the carriage engages the thread on the lower portion of the shaft.

7. Apparatus for electrostatically coating an article comprising a frame, a vertical drive shaft rotatably mounted in the frame and provided with a spiral feed thread having a constant pitch along its upper portion, a progressively smaller pitch along its lower portion and a return thread; gear means for rotating the head including an annular gear having inner and outer teeth; a vertical spline shaft rotatably mounted in the frame and engaging the inner teeth in the annular gear; a pump for supplying coating material to the feed opening; an electric motor for operating the pump; a drive motor connected to the spline shaft for constantly rotating the same; a magnetic clutch for clutching the drive shaft to the driven motor; a manually operable switch for engaging the magnetic clutch; a switch positioned to be operated as the engaging means on the carriage engages the feed thread on the lower portion of the drive shaft to deenergize the pump motor; and a second switch positioned to be operated by the carriage as it is returned to the upper portion of the drive shaft by the return thread to deenergize the magnetic clutch.

8. The method of coating an article by electrostatic depositing of coating material from an atomizing means comprising feeding coating material from a remote control point to the atomizing means, free flowing the mate rial from the area of feed to the atomizing means to the atomizing zone of said atomizing means where said material has an exposed free surface, creating an electrostatic field between the material at the atomizing zone and the article of sufiicient strength as to be capable of electrostatically atomizing coating material at said zone and electrostatically depositing the atomized coating material on the article, causing relative movement between the article and the atomizing means to coat a portion of the article and then reducing the feed of coating material at the control point progressively to reduce the rate of feed to the atomizing zone while simultaneously progressively reducing said speed of relative movement to coat another portion of the article.

9.- The method, of coating an article having an abrupt discontinuity by electrostatic deposition of coating material from an atomizing means comprising feeding coating material from a remote control point to the atomizing means, free flowing the material over a surface of the atomizing means to the atomizing zone thereof where said material has an exposed free surface, creating an electrostatic field between the atomizing zone and the article of sufiicient strength as to be capable of electrostatically atomizing coating material at said zone and electrostatically depositing the atomized coating material on the article, causing relative movement between the article and the atomizing means in the direction toward the discontinuity to coat an intermediate area of the article, and then cutting off the feed at the control point to progressively reduce the feed to the atomizing zone to zero when the atomizing means is near the discontinuity While simultaneously progressively reducing the speed of said relative movement.

10. The method of coating the interior surface of a hollow article by electrostatic deposition of coating material from an atomizing means comprising feeding coating material at a substantially constant rate from a remote control point to the atomizing means, flowing the coating material across a surface of the atomizing means to the atomizing zone thereof where the material has an exposed free surface, creating an electrostatic field between the atomizing zone and the article of sufficient strength as to be capable of electrostatically atomizing coating material at said zone and electrostatically depositing the atomized coating material on the article, moving the atomizing means through the hollow in the article, and then cutting off the feed at the control point to progressively reduce the feed to the atomizing zone as the atomizing means approaches the end of the hollow to produce a feed of substantially zero at the atomizing zone when the atomizing means is closely adjacent the end of the reducing the speed of move to minimize undesired variahollow while simultaneously ment of the atomizing means tions in coating thickness.

11. The method of coating discontinuity by electrostatic rial from an annular atomizing means comprising feeding coating material to the central area of the atomizing means, rotating the atomizing means to flow the coating material over a surface of the atomizing means to the edge thereof, creating an electrostatic field between the coating material at said edge and the article of suflicient strength as to be capable of electrostatically atomizing coating material at said edge and electrostatically depositing the atomized coating material on the article, causing relative movement between the article and the atomizing means to coat a portion of the article, and then cutitng ofi the feed of coating material While continuing the rotation of the atomizing means to cause the coating material remaining on said surface of the atomizing means to flow in decreasing amounts to the edge thereof and while simultaneously progressively reducing the speed of relative movement to bring the atomizing means nearest the discontinuity as the flow of coating material across said surface and to said edge reaches approximately zero.

an article having an abrupt deposition of coating mate- References Cited in the file of this patent UNITED STATES PATENTS 1,812,854 Beach July 7, 1931 1,861,475 Hopkins June 7, 1932 2,083,633 Brackett June 15, 1937- 2,336,946 Marden Dec. 14, 1943 2,438,561 Kearsley Mar. 30, 1948 2,508,509 Germer et a1 May 23, 1950 2,509,448 Ransburg et al May 30, 1950 2,559,225 Ransburg July 3, 1951 2,568,611 Crouse Sept. 18, 1951 

1. THE METHOD OF COATING AN ARTICLE BY ELECTROSTATIC DEPOSITION OF COATING MATERIAL FROM AN ATOMIZING MEANS COMPRISING FEEDING COATING MATERIAL FROM A REMOTE CONTROL POINT OF THE ATOMIZING ZONE OF THE ATOMIZING MEANS WHERE SAID MATERIAL HAS AN EXPOSED SURFACE, CREATING AN ELECTROSTATIC FIELD BETWEEN THE ATOMIZING ZONE AND THE ARTICLE OF SUFFICIENT STRENGTH AS TO BE CAPABLE OF ELECTROSTATICALLY ATOMIZING COATING MATERIAL AT SAID ZONE AND ELECTROSTATICALLY DESPOSITING THE ATOMIZED COATING MATERIAL ON THE ARTICLE, CAUSING RELATIVE MOVEMENT BETWEEN THE ARTICLE AND THE ATOMIZING MEANS TO COAT A PORTION OF THE ARTICLE AND THEN REDUCING THE FEED AT THE CONTROL POINT TO PROGRESSIVELY REDUCE THE RATE OF FEED TO THE ATOMIZING ZONE WHILE PROGRESSIVELY REDUCING THE SPEED OF RELATIVE MOVEMENT TO COAT ANOTHER PORTION OF THE ARTICLE. 